The present invention relates to a brake pressure modulator for use with skid-controlled brake systems of automotive vehicles of the type including at least one volume take-up means provided in the hydraulic conduit leading from a master cylinder to one or more wheel cylinders, with the volume take-up means including a plunger disposed in a cylindrical chamber in communication with the wheel cylinder circuit and preloaded by a force into the cylindrical chamber. The plunger is closely guided outwardly from the cylindrical chamber and is in communication with a vacuum-operated drive mechanism which, upon occurrence of a brake skid signal, partly moves the plunger out of the cylindrical chamber to increase the volume of the cylindrical chamber to thereby correspondingly decrease the brake pressure.
Brake pressure modulators of the afore-described type, upon occurrence of a brake skid signal, counteract the spring which tends to restore the plunger to its resting position with the consequence that, with increasing retraction of the plunger from the cylindrical chamber, the force required to be generated by the driving mechanism must increase, which, if the driving mechanism is made up of a pneumatically operated piston/cylinder arrangement, is unfavorably space-consuming.
Also, it is desirable in such systems to hold the piston in its normal position solely by the force of the driving mechanism and, depending on the amount of brake skid, have the piston retract in a controlled manner. A shortcoming of such system involves the problem that, at the beginning of the retraction movement of the plunger, a very high force corresponding to the high pressure in the wheel brake circuit, will have to be counteracted, whereas, with the plunger more or less moved out, the hydraulic pressure, and thus the force acting upon the plunger, will decrease.